1. Field of the Invention
The present invention relates in general to a hydraulic cushioning apparatus for a press, and more particularly to a hydraulic system which includes hydraulic cylinders linked with respective pressure pins or cushion pins for supporting a workpiece through a pressure pad.
2. Discussion of the Prior Art
Such a hydraulic cushioning apparatus is known, according to laid-open Publication Nos. 2-39622 and 1-60721 of Japanese Utility Model Applications, for example.
The basic arrangement of the hydraulic cushioning apparatus disclosed in the above-identified publications is illustrated in FIG. 4, wherein the cushioning apparatus includes a pressure pad in the form of a cushioning ring 6 disposed adjacent to a lower die 3 of a press. The cushioning ring 6 is supported by a plurality of pressure pins 7 such that the top surface of the ring 6 is substantially flush with the top surface of the lower die 3, so that a workpiece W in the form of a metal strip placed on the ring 6 is protected against wrinkling when the lower die 3 cooperates with an upper movable die 1 to perform a pressing action on the workpiece W.
The cushioning apparatus further includes hydraulic cylinders 8 corresponding to the pressure pins 7. Each of these cylinders 8 has a cylindrical wall 8a linked with a pneumatically operated die cushioning device 9, through a cushion platen 10. Each hydraulic cylinder 8 also has a piston 8b fixed to the lower end of the corresponding pressure pin 7 remote from the cushioning ring 6. The cylindrical wall 8a and the piston 8b cooperate to define a fluid chamber, and the fluid chambers of the cylinders 8 are connected to a common manifold 15, which in turn is connected to a hydraulic power supply 20 via a flexible tube 16 and a check valve 21. The check valve 21 permits a flow of a working fluid in the direction from the hydraulic power supply 20 toward the hydraulic cylinders 8, and inhibits a flow of the fluid in the reverse direction.
The hydraulic power supply 20 is adapted to provide the pressurized fluid having a preset pressure level, so that the fluid is supplied through the check valve 21 and manifold 15 to the hydraulic cylinders 8 when the pressure in the cylinders 8 falls below the preset level. Thus, the pressure in the cylinders 8 is maintained at the preset level. It is noted that the check valve 21 prevents the fluid from flowing from the hydraulic cylinders 8 back to the hydraulic power supply 20, even when the pressure in the cylinders 8 rises above the preset level (pre-load level) when the upper movable die 1 is moved down to force the workpiece W against the lower die 3 and the pressure pad in the form of the cushioning ring 6, during a pressing cycle in which the movable die 1 is reciprocated between an upper stroke end and a lower stroke end, by a suitable drive mechanism.
According to the hydraulic cushioning apparatus constructed as described above, the cushioning pressures of the pressure pins 7 which act on the cushioning ring 6 can be made substantially equal to each other, thereby assuring a pressing operation with high precision, with a uniform cushioning force exerted on the cushioning ring 6 and the workpiece W over the entire working area.
The die cushioning device 9 uses an air cylinder 11 which supports the pressure platen 10 and which is supplied with a pressurized air from a pneumatic pressure source 12, through an air regulator 13 and an air tank 14, as well known in the art.
However, the known hydraulic cushioning apparatus suffers from the following problem. Namely, the pressure pins 7 are subject to an abrupt increase in the load upon collision of the movable die 1 against the workpiece W, during the downward movement of the die 1. The increased load or impact force received by the pressure pins 7 are transmitted to the die cushioning device 9 through the hydraulic cylinders 8.
Generally, the air pressure in the air cylinder 11 and the hydraulic pressure in the hydraulic cylinders 8 upon colliding contact of the upper movable die 1 with the workpiece W are such that the pressure platen 10 is instantaneously lowered due to the impact force indicated above, with the cylindrical walls 8a being lowered a greater distance than the pistons 8b which are lowered with the pressure pins 7. This results in a temporary increase in the volume of the fluid chamber of each hydraulic cylinder 8, whereby the fluid pressure in the fluid chamber is lowered below the preset level, so that the pressurized fluid is fed instantaneously from the hydraulic power supply 20 into the hydraulic cylinders 8 until the pressure in the cylinders 8 rises to the preset level of the power supply 20. This phenomenon, so-called "pumping action", causes the pressure in the cylinders 8 to be higher than the preset level when the upper movable die 1 returns to the upper stroke end, that is, when the pressing cycle involving a reciprocation of the die 1 between the upper and lower stroke ends is completed.
With the "pumping action" repeated with the pressing cycles, the pressure in the hydraulic cylinders 8 at the beginning of each pressing cycle gradually increases from the preset level, causing considerable deterioration of the cushioning function of the cylinders 8 and its function of compensation for possible variation in the length of the pressure pins 7, and eventually leading to leakage of the fluid from the cylinders 8 and/or damaging of the cylinders or other troubles with the hydraulic system.